In general, a CNC machine is extensively used in a mechanical manufacturing field, and CNC machines can perform different manufacturing processes such as drilling, rolling, boring and milling a hole of a workpiece, and a milling cutter of the CNC machine is usually fixed onto the main shaft of the machine by a knife holder, and an inclined plane at the front end of the knife holder is used for holding and fixing the knife to the machine, so that the knife can be installed onto the machine.
The advantage of using an inclined plane resides on that it does not require a precise design of the knife holder to fit a knife of various different machines, and the tapered inclined plane is used for progressively sheathing the knife into the machine to hold and fix the knife automatically for a better installation condition. Therefore, such arrangement is applicable for different machines. Even if there is a discrepancy of the manufacturing precision of the machine or knife holder, the problem can be overcome easily, and the knife holder can be mounted securely onto the machine.
However, the machine and the knife holder of such design are held by the inclined plane of the knife holder only. Unless the machines and knife holders come with a special specification (which are usually expensive) that fit the design, the distal surface of the main shaft is not in contact with the knife holder, and the knife holder cannot truly press against the main shaft, such that when the machine is run, the knife holder will be shaken and the manufacturing precision will be affected adversely.
U.S. Pat. Nos. 5,352,073 and 6,739,810 disclose remedial measures for overcoming the shortcomings of the foregoing traditional designs, and present commercialized technologies also change the design of the holding position of the machine by adding a pressing portion protruded from the surface of the main shaft, such that when the knife holder is held, the pressing portion presses against the knife holder in a so-called “dual contact” fashion to improve the stability of the manufacture. However, this method changes the structure of the machine, and users have to send the machine to the original manufacturer for changing the components. Thus, manufacturers have to shut down the production line during the period of making changes to the machine, and thus causing tremendous trouble to manufacturers in addition to an increase of cost.